This invention relates to a real time SAR processing system for processing synthetic aperture radar return signals and a method for real time processing of SAR return signals.
Synthetic aperture radar (SAR) is a technique widely used in aerial and space reconnaissance. An aircraft or a spacecraft carries a side looking antenna and transmits radar pulses in a direction different from the flight path. A slant range coordinate is defined in a direction normal to the flight path and an azimuth coordinate is defined in the direction along the flight path. The range resolution of the SAR is a function of the effective transmitted pulse width, and a high resolution in range is achieved by the use of very short transmitted pulses and/or using chirped pulses. The azimuth resolution is set by the dimensions of the antenna. By increasing the antenna diameter, the azimuth resolution can be increased, being limited, however, by the size and weight of the antenna carried by the aircraft or spacecraft. In conventional radar systems, especially at long ranges, high resolution in azimuth would require the need for huge antennas.
In synthetic aperture radar, the radar platform moves along a straight path in a direction oblique to the target to be imaged. The antenna is carried along the flight path to a sequence of positions in which the SAR system, at each position, radiates a pulse and receives and stores the reflected return signal. SAR images are the superposition of many backscatter pulses within the range and azimuth of the SAR antenna footprint. The stored data is then processed to create the image of the illuminated target area. A high resolution in the azimuth direction is attained by applying a specialized signal processing technique without the necessity of physically large antennas. In effect, a large aperture antenna is synthesized. Since the high resolution in azimuth is achieved by the synthetic antenna and a coherent processing of the phase history, the amplitude and phase history of the reflected signal of a scene has to be recorded. Thus, a coherent phase history of the pulse return signal is generated and recorded. An overview and details of SAR is given in “Tutorial Review of Synthetic Aperture Radar (SAR) With Applications to Imaging of the Ocean Surface” by Kiyo Tomiyasu, Proceedings of the IEEE, Vol. 66, No. 5, May 1978, which is included in this application by reference.
SAR signal processing can be mathematically described as a correlation or a filtering process on all the coherent radar signal returns stored during an (synthetic) aperture time. This requires large data storage and huge data processing capabilities. In the beginning of SAR technology, the computer technology was not powerful enough to process the data amount of SAR images effectively. Therefore, mostly optical processing solutions of the SAR images have been applied. In a conventional optical processing system, the raw SAR data is recorded on a photosensitive material (film) for storage and subsequent optical processing. A reflective range pulse from the radar receiver is written, by means of a cathode ray tube, as a range trace across the width of the signal film. Between pulses the film is advanced by small increments. Thus, a position across the film width corresponds to a range position, and a position along the film length corresponds to a position along the track of the radar platform. The signal amplitude and the phase history are recorded on film after phase coherent addition of the received signal and a reference signal of the radar system. The film is recorded during acquisition of the image and must be (chemically) developed before providing the input for the optical processor for the imaginary construction. This prevents real-time processing and requires chemical film treatment that usually cannot be performed in an aircraft or a spacecraft.
Due to the large amount of data an electronic signal processing of the radar return signals depends on high performance computers and, therefore, an onboard processing on satellites is usually not possible. Furthermore, the large amount of data acquired cannot be effectively compressed so that the large amount of uncompressed data needs to be transmitted from a satellite to a ground based processing centre for further processing. This requires large bandwidth for the data transmission.
In order to allow a real-time processing of synthetic aperture radar signals, U.S. Pat. No. 4,929,953 proposes to optically store raw SAR data on erasable and reusable photosensitive materials. The SAR return signal is written in radial traces on a photosensitive rotatable disk that rotates in synchronicity to the aircraft velocity. The recorded SAR data of several radar pulses is then illuminated by coherent light for the optical processing to reconstruct the SAR image. Next, the disk with the data stored thereon rotates into an erase zone, where the data is erased by high intensity light that changes the recording medium back to its original transparent condition ready for recording of another radial trace of SAR return data. Due to the movable recording medium, which needs to be rotated in exact synchronism with radar pulses and flight parameters, a complex control of the disk rotation is necessary. In addition, the recording of the raw SAR data in radial traces requires a complex shaped aspherical lens for compensation of the radial geometry of the recording medium. Furthermore, due to the moving parts and the thermoplastic recording media, the durability of the proposed real-time SAR processing system is limited so that it is not suitable for a permanent application in a spacecraft.
It is an object of the present invention to provide a real-time SAR processing system and method with reduced data processing requirements and enhanced durability.
This object is achieved by the subject-matter according to the independent claims The dependent claims refer to preferred embodiments of the invention.